One of the most common questions asked of veterinarians is how often pets need to have a heartworm test. Both the American Veterinary Heartworm Society and most veterinarians do recommend yearly heartworm testing. Even though many animal guardians keep their pets on year-round heartworm medication, it is still important to test pets yearly for several reasons.
One of the most important reasons for an annual heartworm test is that, even though most products are highly effective in preventing heartworm infection, there is still no product that is 100% effective. I have seen many cases in my 20-year veterinary career of pets on year-round heartworm preventative medication that have tested positive for heartworms. There has also been increasing recent concern about developing heartworm resistance to some of the older products, and until this issue is sorted out, it is still best to have all pets tested every year. Also in the possibility that a pet is heartworm positive, there can be very occasional severe and even rare fatal reactions should a pet that is positive for heartworms continue to receive monthly preventative.